Pascal Wassong has a retroanalysis column in a french magazine devoted to chess
problems : Phénix. He helped me with this column. He mentioned other
magazines: The Problemist - GB, StrateGems - USA, and Problem Paradise - JP.
"There is a mailing list for retro problems at retros@janko.at. Otto Janko
is the moderator." Here is the most extreme retrograde problem I know of.
Tom Volet (Stratagems) brought it to my attention.

What were the last 96 moves?
Gyula Breyer (1922) and Gerd Wilts (1991)

In the first problem below, the trick is to figure out Black's last
move. Obviously, the black king must have moved from a7, moving out
of check. But how did White deliver the check? With some analysis,
you can see that White moved a knight from b6 to a8. Thus, the last
move was for the Black King to take a White Knight on a8. This idea
of 'unplaying' is a common theme in retrograde analysis. These first
two problems are fairly easy.

These next to problems are slightly trickier, but still on the easy
side.

Color the pieces 1966-1971
Gideon Husserl, Israel Ring Tourney

Each side has made exactly four moves. How?
Tibor Orban, Die Schwalbe, 1976

The next two problems are quite hard. The first was sent to me
by Harry Nelson. [This problem had a cook -- a pawn was added at
g2 to fix it.] Note that the second problem is in a game of U-Chess.
All the moves were in three characters of English notation (P-K3, B-R4,
etc), and none of these moves was ambiguous.

How far can you 'unplay' from this position?
Karl Scherer, first publication

U-Chess.
Show that either side may castle, but not both.
Mannis Charosh, Fairy Chess Review, Dec 1953

These last two problems are also quite hard, but well worth the effort.

What was the first move of the black King?
Dr. Luigi Ceriani, 1952.

Each side has made exactly twelve moves. How?
Gianni Donati, Die Schwalbe, 1998, Dedicated to Danna.